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NEC - Conductors and protection for air conditioning equipment (Based on the 2011 NEC

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    #31
    Originally posted by augie47 View Post
    When we calculate loads for a warehouse at 1/4 va per sq foot we "round down" that result. Should this be different ?
    This is a case which shouts out attention to Mike's comment in this training PDF about just when you get to apply the rounding rule.
    220.5 does not allow us to drop fractions below .5 in general, just in the case of Amps.
    So, I am going to calculate the load for a 4500 sq. ft. warehouse.
    1. One order of calculation gives me 1125 VA. Dividing by 120V, I get 9.375A and round down to 9A. No questions about that number.
    2. But 1125 VA = 1.125 kVA. So I round down to 1.0 kVA, and then get 8.33A, which I round down again to get 8A. I am not actually allowed to do this since I am only allowed to round Amps, not VA or kVA (or KA, for that matter!)
    3. OK. Take a different look at it: 1/4 VA per square foot. I don't like working with that, so I will convert it to Amp per square foot at 120 volts. That works out to be .00208 Amps per square foot. That is so small that I can clearly round it down to zero. So 4500 sq. ft. times 0 = 0 Amps calculated load.
    4. Or the warehouse is divided into two sections, one of 2000 sq. ft. and one of 2500. Taking them separately, I get 4.17 for the first part and 5.208 for the second. I round both down and get 4 and 5, then add to get 9 amps. That happens to work out the same.
    5. Now let's try four sections, of 1195, 1195, 1195, and 915. That gives us 2.49, 2.49, 2.49 and 1.9. Rounding, we get 2, 2, 2, and 2, for a total of 8 Amps again.

    It actually can make a difference if you need to multiply and divide several times during an overall calculation or to add two or more numbers, each of which was calculated. The safe (and sane) thing to do is to wait until the calculation has finally produced an Amp number that you need to apply directly before doing the rounding. But the code does not actually say that.
    Last edited by GoldDigger; 03-17-13, 02:52 PM. Reason: Tidied it up.

    Comment


      #32
      Originally posted by GoldDigger View Post
      This is a case which shouts out attention to Mike's comment in this training PDF about just when you get to apply the rounding rule.
      220.5 does not allow us to drop fractions below .5 in general, just in the case of Amps.
      So, I am going to calculate the load for a 4500 sq. ft. warehouse.
      1. One order of calculation gives me 1125 VA. Dividing by 120V, I get 9.375A and round down to 9A. No questions about that number.
      2. But 1125 VA = 1.125 kVA. So I round down to 1.0 kVA, and then get 8.33A, which I round down again to get 8A. I am not actually allowed to do this since I am only allowed to round Amps, not VA or kVA (or KA, for that matter!)
      3. OK. Take a different look at it: 1/4 VA per square foot. I don't like working with that, so I will convert it to Amp per square foot at 120 volts. That works out to be .00208 Amps per square foot. That is so small that I can clearly round it down to zero. So 4500 sq. ft. times 0 = 0 Amps calculated load.
      4. Or the warehouse is divided into two sections, one of 2000 sq. ft. and one of 2500. Taking them separately, I get 4.17 for the first part and 5.208 for the second. I round both down and get 4 and 5, then add to get 9 amps. That happens to work out the same.
      5. Now let's try four sections, of 1195, 1195, 1195, and 915. That gives us 2.49, 2.49, 2.49 and 1.9. Rounding, we get 2, 2, 2, and 2, for a total of 8 Amps again.

      It actually can make a difference if you need to multiply and divide several times during an overall calculation or to add two or more numbers, each of which was calculated. The safe (and sane) thing to do is to wait until the calculation has finally produced an Amp number that you need to apply directly before doing the rounding. But the code does not actually say that.
      Basic rules of mathematics are that you only round at the end of your calculations. (But then again, you would need to KNOW that in order to understand why the NEC isn;t specific about it. eg. they are not into creating new math.)

      Comment


        #33
        Originally posted by weressl View Post
        Basic rules of mathematics are that you only round at the end of your calculations. (But then again, you would need to KNOW that in order to understand why the NEC isn;t specific about it. eg. they are not into creating new math.)
        It is quite clear when your entire goal is to get one end result number.
        It is less obvious when you are actually using the intermediate numbers themselves and then want to add them up later for some other purpose. In that case you have to instead go back to the beginning (before rounding) and add them up, or else start the whole calculation over if you did not save the unrounded numbers.
        You do not necessarily need to know Math (rather than just Arithmetic) to use the NEC, but it sure helps! It also helps to know Logic, but sometimes that can get you into trouble too.
        Or, as Mike jokingly stated in my reference, where you round depends on the result you want to get.

        Comment


          #34
          Originally posted by GoldDigger View Post
          It is quite clear when your entire goal is to get one end result number.
          It is less obvious when you are actually using the intermediate numbers themselves and then want to add them up later for some other purpose. In that case you have to instead go back to the beginning (before rounding) and add them up, or else start the whole calculation over if you did not save the unrounded numbers.
          You do not necessarily need to know Math (rather than just Arithmetic) to use the NEC, but it sure helps! It also helps to know Logic, but sometimes that can get you into trouble too.
          Or, as Mike jokingly stated in my reference, where you round depends on the result you want to get.
          Her is one example of errors created by immediate rounding:

          (First number is multiplied by Pie, second is sqrt multiplied by the number itself and the third is the sin product of the number. Finally the three numbers are multiplied with each other. Produces 76.7.... In the second round the same sequence is repeated EXCEPT the base numbers are rounded, producing 58.1... a VERY different number.)
          Base #
          3.123 9.811194
          4.567 9.759926
          32.345 0.801066
          76.70727
          3 9.424778
          5 11.18034
          32 0.551427
          58.10505

          Comment


            #35
            Mr. Weress:
            Is it logical to expand that to the case at had and say that in looking at the load for the this one HVAC unit, we drop the 0.1 and allow a conductor with a ampacity of 30, but, in the event we have 15 units, we would look at a total load of 451.5 amps as opposed to 450 ?

            (For discussions sake I am only talking load...ignoring any multipliers for conductor ampacity requirements, etc)
            At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

            Comment


              #36
              Originally posted by augie47 View Post
              Mr. Weress:
              Is it logical to expand that to the case at had and say that in looking at the load for the this one HVAC unit, we drop the 0.1 and allow a conductor with a ampacity of 30, but, in the event we have 15 units, we would look at a total load of 451.5 amps as opposed to 450 ?

              (For discussions sake I am only talking load...ignoring any multipliers for conductor ampacity requirements, etc)
              It is not an issue of logic, it is rules of arithmetic. BTW it would be 30 and 452.

              Comment


                #37
                Quick show of hands: who wishes Mike's MCA was 30.6 at this point?

                Comment


                  #38
                  Originally posted by George Stolz View Post
                  Quick show of hands: who wishes Mike's MCA was 30.6 at this point?
                  Sure would have made life simpler
                  At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

                  Comment


                    #39
                    Originally posted by George Stolz View Post
                    Quick show of hands: who wishes Mike's MCA was 30.6 at this point?
                    Not me.

                    I love it when the pros on this forum go at it.
                    Good info(or just different ways of looking at things)for the rest of us.

                    Comment


                      #40
                      Last edited by hurk27; 03-17-13, 10:16 PM.
                      Wayne A. From: N.W.Indiana
                      Be Fair, Be Safe
                      Just don't be fairly safe

                      Comment


                        #41
                        Originally posted by hurk27 View Post
                        I'm a little confused at the response of saying that a #10 wire can't be used for a 30.1 amp rating when used for a 240.4(G) purpose?

                        Comment


                          #42
                          Originally posted by augie47 View Post
                          Don,
                          I keep seeing it both ways. Until this thread, I never dropped fraction of an ampere if the figure was less than 0.5 except when calculations with Art 220 itself.
                          I can see validity based on 220.1, "220.1 Scope. This article provides requirements for calculating branch-circuit, feeder, and service loads." that Art 220 does apply to branch circuits.
                          220.3 notes that other Articles may be in addition to or modify the values in Art 220.
                          220.5 allows us to drop fractions less than 0.5.
                          The 30.1 is a "calculated load", calculated by the manufacturer, but nevertheless calculated and still under the overall scope of Art 220.
                          When we calculate loads for a warehouse at 1/4 va per sq foot we "round down" that result. Should this be different ?

                          As with many Code sections, the wording certainly could be modified to remove any ambiguity. Until that time this one will remain controversial.
                          Gus,
                          I just don't see the calculation for the minimum ampacity of a conductor as a load calculation. I don't see Article 220 as having anything to do with any wire size calculation.
                          Don, Illinois
                          (All code citations are 2017 unless otherwise noted)

                          Comment


                            #43
                            Originally posted by don_resqcapt19 View Post
                            Gus,
                            I just don't see the calculation for the minimum ampacity of a conductor as a load calculation. I don't see Article 220 as having anything to do with any wire size calculation.
                            Well no not directly. But to know minimum ampacity necessary for a conductor you do need to know how much load it will be carrying.
                            I live for today, I'm just a day behind.

                            Comment


                              #44
                              Originally posted by don_resqcapt19 View Post
                              Gus,
                              I just don't see the calculation for the minimum ampacity of a conductor as a load calculation. I don't see Article 220 as having anything to do with any wire size calculation.
                              I respect that. As I stated earlier I think there is ambiguity in the wording.
                              Obviously, at the moment, I am leaning toward the "round down" camp but I certainly don't feel my feet are on as firm a platform as I would prefer. I'm definitely going to look closer although I doubt I will have an iron-clad conclusion.
                              When I[m confronted with the situation, it will probably fall into the "some folks feel category" which I dislike but have occasionally adopted.
                              At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

                              Comment


                                #45
                                Originally posted by augie47 View Post
                                I respect that. As I stated earlier I think there is ambiguity in the wording.
                                Obviously, at the moment, I am leaning toward the "round down" camp but I certainly don't feel my feet are on as firm a platform as I would prefer. I'm definitely going to look closer although I doubt I will have an iron-clad conclusion.
                                When I[m confronted with the situation, it will probably fall into the "some folks feel category" which I dislike but have occasionally adopted.
                                I think the real problem here is that the rounding permission is in Art.220. As Don said, Art. 220 is for calculating load-not ampacity. George's idea to put elsewhere in the code and make all inclusive would solve that.
                                As an example, Art 220 and 240 refer you to Art. 430 for motor applications. I don't see anything in Art 430 to indicate what to do with fractions of amp. The same is true for the A/C unit in mike's video-Art 440 is silent on rounding for ampacity.
                                Like you, I'm just not sure what the answer is. One thing for sure, Mike seems pretty confident in his answer.

                                Comment

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